1. Field of the Invention
The present invention relates to an absolute encoder, a signal processing method, a program, a driving apparatus, and an industrial machine.
2. Description of the Related Art
An absolute encoder is capable of reading a mark string corresponding to a binary code such as a gray code and an M sequence code by using a light receiving element array to output absolute position information. Some absolute encoders may use a plurality of mark strings while other absolute encoders may use a single mark string. An absolute encoder using a plurality of mark strings may be based on a scheme for simultaneously reading mark strings on a plurality of tracks on a scale, including a mark string corresponding to a gray code string. This scheme that simultaneously reads a plurality of tracks may require precise attachment of a head and a scale of an encoder with a predetermined positional relationship therebetween, which makes it difficult to achieve both a higher resolution and higher accuracy.
A scheme has been known which has a single mark string on a scale while attempting a higher resolution (Japanese Patent Laid-Open No. 2012-37392). This scheme uses a scale in which a plurality of marks including a plurality of types of marks are arranged with a space and a period. Furthermore, a detector is used which includes a plurality of photoelectric conversion elements arranged along the direction of the scale and detects a plurality of marks of a part of the scale with the photoelectric conversion elements. Then, an amplitude of each period of periodic signals for a plurality of periods output from the detector is quantized to generate a data string. Then, first position data is generated from the data string. The first position data indicates an absolute position along the arrangement of scales or detectors and handles the period as a resolution. Furthermore, second position data is acquired from a phase of a signal for at least one period of the periodic signal. The second position data relates to a section acquired by dividing one period as a resolution. Then, the first position data and the second position data are related as a higher-order bit and a lower-order bit to form data that describes an absolute position.
An encoder based on the scheme according to Japanese Patent Laid-Open No. 2012-37392 has two types of amplitude occurring in a periodic signal if two types of marks are included. Therefore, the two types of amplitude are quantized (binarized) to acquire a binary data string. However, when the attachment state of at least one of a head and a scale of the encoder changes, a signal acquired from a detector may be distorted or may have fluctuating offsets. When a characteristic (such as light transmissivity) of a mark changes, two types of amplitude in a periodic signal acquired from two types of marks may change. Thus, when a fixed threshold value is used for the quantization, an accurate data string indicating an absolute position such as an M sequence code may possibly not be acquired.